42 ON THE SPECTRUM OP THE OXY-flYDROGEN FLAME. 



by which two or three lines fall nearly in the same place, and produce a hazy band in 

 the photograph which cannot be resolved into its constituent lines. We thiuk, there- 

 fore, that the groups do actually follow the law above enunciated. M. DESLANDRES 

 has observed the same law to hold in regard to inverse wave-lengths in the lines 

 forming the bands of nitrogen, in those of A, B, and a, and in the groups of several 

 spectra of compound gases ; and has remarked that it is the law of sequence of the 

 harmonics of solid rods. A. HERSCHEL and PIAZZI SMYTH had previously noticed this 

 law in the sequence of the rays in one of the bands of the spectrum of carbonic 

 oxide. 



While the work of photographing and measuring this spectrum was in progress 

 we received from Dr. GRUNWALD, Professor of Mathematics in the I. R. German 

 Technical High School at Prague, a long list of lines which he had, on theoretical 

 grounds, predicted would be found in the spectrum of water. The interest attaching 

 to this prediction induced us to make a more extended and complete investigation of 

 the spectrum than we had originally intended. Many of these predicted lines, 

 though not all of them, agree closely with lines which we have recorded in the 

 spectrum of the oxy-hydrogen flame ; and in the weaker parts of the spectrum at 

 either end, Dr. GRUNWALD'S list includes many of the strongest lines. His results 

 have been (in part) published in the ' Astronomische Nachrichten.' We are not at 

 present in a position to discuss his theory, which is a far-reaching one, and will need 

 to be tested at many points ; but the coincidences between his predictions and the 

 lines observed are very remarkable, and will, no doubt, attract the attention of many 

 besides ourselves. 



A map of the spectrum on a scale of inverse wave-lengths accompanies this Paper, 

 and also an enlarged photograph which gives the general characters of the spectrum, 

 though in the strongest part it has been over-exposed. The photograph includes 

 the regions of the lines enumerated in Tables III. to VI., and greater part of VII. 



